How to Make It
Preheat oven to 425°F. Combine apples, oil, cinnamon, and 1/4 teaspoon of the salt on a half-size (18- x 13- x 1-inch) rimmed baking sheet; toss to coat. Bake in preheated oven until apples are tender and beginning to brown, 15 to 20 minutes, stirring after 10 minutes.
Process milk, eggs, vanilla, and remaining 1/4 teaspoon salt in a blender until smooth, about 30 seconds. Whisk together flour and granulated sugar in a large bowl. Add flour mixture to milk mixture in 3 additions; process until smooth after each addition.
Drizzle melted butter over apple mixture; stir to combine. (Make sure butter completely coats bottom of pan.) Pour batter evenly over apple mixture. Bake at 425°F until puffy and golden, 15 to 20 minutes.
Sprinkle evenly with powdered sugar. Cut into squares, and serve immediately.